As is well known, the operation of an electrophotographic system, which has been widely used in copying machines, printers and printing machines, generally includes the following steps of:    (1) charging a photoconductive material;    (2) exposing the photoconductive material (formation of a latent image);    (3) developing the latent image with a developing agent (formation of toner image);    (4) transferring the toner image onto a recording medium; and    (5) fixing the transferred toner image.
In the final fixing step (5), a heat roll fixing system which applies pressure to the toner by means of a heated fixing roller thereby to melt and fix the toner, and a flash fixing system which melts and fixes the toner by irradiating it with light such as flashlight are used.
The heat roll fixing system has such advantages that a cheap apparatus can be provided because the toner is fixed at high temperature under pressure and also the toner surface becomes smooth by pressing with a roller thereby making it possible to enhance the printing density. On the contrary, this fixing system has disadvantages in that the recording paper is curled due to high temperature after the fixation and an offset occurs because the fixing roller is stained with the toner and also has disadvantages in that it is difficult to achieve high speed operation due to curling of the paper and the toner is difficult to fix on a sealed post card whose surface is coated with glue.
On the other hand, the flash fixing system has various noticeable merits, although it has one problem in that a flash tube used as a light source has to be provided at a low price. The typical merits are as follows.
(1) The toner can be fixed on a recording medium without making contact because the toner is melted by exposure to light, and therefore curling of the recording paper and offset are eliminated, although the flash tube used as the light source is expensive, and therefore neither staining of the image (offset) during the contact fixation nor reduction of the resolution (spread of the image caused by passage through upper and lower rollers) is recognized.
(2) The degree of freedom with respect to design of the toner and fixing equipment is large because a specific design for prevention of stain of the image recognized during the contact fixation (for example, addition of wax as a releasant to the toner, application of silicone oil as a releasant onto the fixing roller or the like) is not required.
(3) Fixation can be achieved regardless of the kind (for example, material and thickness) of the recording paper to be used.
(4) High-speed recording can be achieved because neither curling of the recording paper nor offset occurs after the fixation, as described above, and the image can be fixed easily on a special recording paper such as a sealed post card.
In view of many advantages as described above, the flash fixing system has widely been employed in high-speed printers for business use and high-speed copying machines.
Describing the flash fixing system in more detail, hitherto, various electrophotographic toners have been specially designed and provided for flash fixation. For example, Japanese Unexamined Patent Publication (kokai) No. 5-107805 (corresponding to U.S. Pat. No. 5,330,870) discloses an electrophotographic toner for flash fixation which is less likely to cause odor, white smoke or the like due to decomposition and is capable of fixing without forming voids. This developing composition is characterized in that a polyester resin used as a binder resin is obtained from an acid component, 80 mol % of which is composed of a phthalic acid dicarboxylic acid, and an alcohol component, 80 mol % or more of which is composed of bisphenol A alkylene oxide adduct. Japanese Unexamined Patent Publication (kokai) No. 7-72657 discloses a toner, for an image forming apparatus, which is superior in flash fixability, environmental stability and void resistance. This toner is characterized in that a molecular weight distribution of a polyester polymer used as an essential constituent component has a plurality of molecular weight peaks. A similar toner is also disclosed in Japanese Unexamined Patent Publication (kokai) No. 8-123070. This toner for flash fixation is characterized in that it contains, as an essential constituent component of the toner, polyester and polyether, or polyester modified with polyether. Japanese Unexamined Patent Publication (kokai) No. 8-87128 discloses a toner for flash fixation, which is capable of achieving flash fixability and void resistance at the same time and is less likely to generate a fixation odor. This toner is characterized in that it contains, as a toner binder, a crosslinkable polyester resin using trimellitic acid and epi-bis type epoxy in combination as a crosslinking component, a number-average molecular weight of the resin being within a range from 2,000 to 4,000, a ratio of a weight-average molecular weight to a number-average molecular weight being within a range from 10 to 25.
As is understood from the above descriptions, it is important for the toner for flash fixation to achieve the flash fixability and void resistance at the same time and to eliminate or reduce an odor generated during the fixation. Therefore, a trial of improving the composition of the polyester resin used as the binder resin has been made to solve these problems in the conventional toners.
It has also been known to use a toner comprising a low viscosity polyester resin free from a chloroform-insoluble content in order to improve the flash fixability. However, when using such a toner, white defects (fine white dotted patterns) peculiar to flash fixation, which are called “voids”, occur. The flash fixing system has a problem that, since the temperature of the toner surface is raised to 500° C. upon flash exposure, a low-molecular weight component included in the toner, which is liable to be sublimated, scatters thereto thereby to stain the inside a printing apparatus, resulting in clogging of a desmoking/deodorizing filter attached to the printing apparatus. In the conventional printing apparatuses, smoke is removed by attaching a desmoking/deodorizing filter made mainly of active carbon as a principal component in the vicinity of the fixation portion. However, current commercially available filters must be replaced frequently by a new filter because of their short lifetime. In a toner for flash fixation, an improvement in grinding efficiency in the preparation of the toner is required in view of stabilization and cost reduction of the developing agent. When using the toner in combination with the carrier, prevention of filming on the carrier is also required. If filming of the toner on the carrier can be prevented, it becomes possible to provide a developing agent stable for a long period.